AN2620 Datasheet, PDF (1/20 Pages) STMicroelectronics – 3 A high-frequency synchronous 900 kHz

AN2620
Application note
3 A high-frequency synchronous 900 kHz
step-down converter based on the ST1S10
Introduction
The ST1S10 is a step-down DC-DC converter with an optimized inhibit function for powering
high-voltage LCD applications and low-voltage digital core HDD applications. Generally, it
replaces the high current linear solution when high power dissipation is a problem. It
provides up to 3 A over an input voltage range of 2.5 V to 18 V and synchronous rectification
saves the external Schottky diode. A high internal switching frequency (0.9 MHz) allows it to
use tiny surface-mount components, as well as the resistor divider, to set the output voltage
value. Only an inductor and 3 capacitors are required. The current PWM mode architecture
and stable operation with low E.S.R SMD ceramic capacitors results in low, predictable
output ripple. To maximize the power conversion efficiency in light load, the regulator can
work in burst mode automatically. The device can operate in PWM mode at a fixed
frequency or synchronized to an external frequency. It switches at a frequency of 900 kHz
when SYNC is connected to ground or a fixed voltage (less than 5.5 V) and synchronizes
the switching frequency between 400 kHz to 1.2 MHz from the external clock that is applied
to SYNC. A thermal shutdown circuit is integrated and activates at 150 °C. Cycle-by-cycle
current limitation provides protection against shorted outputs. The on-chip 260 µs power-on
reset ensures the proper operation when switching on the power supply. The quiescent
current is less than 6 µA in the inhibit state. The device is available in MLP4x4 and SO-8
ePad packages.
Figure 1. Simplified schematic
May 2010
Doc ID 13883 Rev 3
1/20
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